The present invention relates to an injection-blow-molded nursing bottle mainly formed of a polyether sulfone (viz., xe2x80x98PESxe2x80x99) resin.
Nursing or feeding bottles made of plastics are often carried by users for use at any desired place, so that for assurance of safety to suckling babies, polycarbonate (viz., xe2x80x98PCxe2x80x99) resins highly resistant to shock have widely been employed as a material for forming those nursing bottles. The PC resins are transparent and, of such an excellent heat-resisting property that articles formed of them can be used within a wide range of temperatures, enabling the boiling of said articles for sterilization. Owing also to these further merits, the PC resins are particularly preferable materials of those nursing bottles. Each polycarbonate macromolecule comprises a main linear chain having therein ester groups (viz., xe2x80x94Oxe2x80x94Rxe2x80x94OCOxe2x80x94), so that glycols and/or any dibasic phenols have been used to react with phosgene or any carbonates. xe2x80x98Bisphenol Axe2x80x99 is a typical example of the dibasic phenols constituting the polycarbonates.
However, bisphenol A involved in the ordinary carbonates has recently been indicated as one of environmental hormones having a pseudoestrogenic effect. Recently, elution of bisphenol A (that is the environmental hormone) out of articles contacting foods, beverages or the like and made of polycarbonates has been reported to exceed an allowable limit.
Particularly, the industries manufacturing nursing bottles for suckling babies are concerned of the problem of environmental hormones. Therefore, urgent efforts have been made to seek an alternative material that will be comparable with polycarbonates in anti-shock property (viz., shock resistance) and heat resistance but nevertheless free from such a hormone problem.
To meet this requirement, polyether sulfones (viz., xe2x80x98PESxe2x80x99) were proposed to substitute for polycarbonates. This resin is one of highly heat-resisting thermoplastic resins, and may be synthesized by polycondensation of dichlorodiphenyl sulphone whose chemical structure is shown in FIG. 6. molecular formula.
Generally speaking, a richer content of phenyl groups provides stiffer macromolecules highly resistant to heat. Polyether sulphone shown in FIG. 6 will be more heat-resisting than ordinary polysulphones. Those polyether sulphones lacking in alkyl groups are of an improved thermal stability and of a higher resistance to oxidation. They are relatively stiff and hard resins of a higher tenacity, having a specific gravity of 1.37, a tensile strength of 860 kgf/sq. cm, an breaking elongation of 40-80%, a bending strength 1320 kgf/sq. cm, a bending modulus of 26500 kgf/sq. cm, and a (Rockwell) hardness of M88.
Unfortunately, PES resins are considerably inferior to PC resins in anti-shock property. In any case, it may be possible to injection-blow-mold any selected PES resin into nursing bottles of the same shape as those which have been made of the PC resin. However, such PES nursing bottles have not proved sufficient in their anti-crash strength in the case of being occasionally dropped to fall onto hard floor, hard ground or the like, thus failing to be used practically.
An increased overall wall thickness may possibly be useful in improving strength. In a case of such resin nursing bottles each for instance of a capacity of about 300 milliliter or less, their bodies will have to be designed to weigh 40 grams or less taking into account productivity, gross weight of each bottle assembly and total manufacture cost. To satisfy these conditions, a maximum overall wall thickness of the bottles will be 1.5 mm, with a minimum of about 0.5 mm. The present inventors have conducted a series of falling tests on bottle samples prepared along this line, to find that PES nursing bottles of the prior art structure did not show any satisfactory anti-crash strength despite their wall thickness of 1.5 mm.
Accordingly, an object of the present invention is to provide an injection-blow-molded nursing bottle that is formed of a PES resin and having its bottom of such a specially modified configuration as affording a sufficient falling impact strength.
An injection-blow-molded nursing bottle provided herein and formed of a polyether sulfone resin is of a generally cylindrical shape and has a closed bottom, wherein the bottom having at a central portion thereof a gate flash generated when the resin is previously injected to prepare a parison, with the central portion being of a convex shape to bulge up towards a mouth of the bottle. A height of such a convex bottom is from 9% to 20%, and more preferably from 10% to 15% of an inner diameter that is possessed by of a generally circular floor-engaging rim of the bottom in a bottom plan view.
The injection-blow-molded PES nursing bottle may be produced by making at first the parison composed of the polyether sulfone resin and having a closed bottom, with use of an injection mold that has a gate at a location corresponding to the central portion of said bottom, and then stretching-blow molding the parison to form the bottle whose central portion is of a convex shape to bulge up or be curved upwards towards a mouth of the bottle. A height of such a convex bottom may be designed to be from 9% to 20%, and more preferably from 10% to 15% of an inner diameter that is possessed by of a generally circular floor-engaging rim of the bottom, in a bottom plan view thereof.
The term xe2x80x98injection-blow moldingxe2x80x99 is meant to include various possible processes by which a parison is injected at first and it is subsequently blow molded. The so-called xe2x80x98stretching-blow moldingxe2x80x99 process falls within this scope, which also includes the cold parison method and the hot parison method. In general, the parison is substantially of a cylindrical shape, and the material employed in the described molding process may contain any additives provided that its main ingredient is a PES.
Such a centrally-raised bottom of the bottle enables it in the invention to effect uniform stretch in all the directions and all around the rim or circular corner of said bottom. Thus improved are the shock resistance and the falling strength of the rim portion, conveniently to users of this nursing bottle. The bottom central portion protrudes towards the mouth a distance equal to or more than 9% of the inner diameter of said rim, that is an actually floor-engaging xe2x80x98bottomxe2x80x99. Even if the nursing bottle containing any amount of milk would slip down to possibly result in damage or crash of the bottom rim due to gravitational shock, the bottom center having the gate flash will never collide with the floor. Residual stress resulting from the injection process and concentrated in such a bottom center will never initiate any instant and centrifugal spreading of breakage. It has been noted that an excessively raised bottom center had often caused a content (such as a breast milk or an artificial milk) to foam or bubble when being poured into the bottle. Also from a viewpoint of appearance and commercial value of the products, i.e., nursing bottles, their raised bottom centers have been restrained to be as low as possible. The bottle of the invention is of a sufficient falling impact strength notwithstanding usage of a PES resin as its material, and the convex bottom center raised by 20% or less avoids the foaming of bottle content and at the same time affords an unimpaired appearance.
In the stretching-blow-molding process, the PES parison will uniformly be heated at first to a temperature below its melting point but above its glass transition temperature (i.e., softening point), before stretching-blow molding the parison in a cold blow mold. This method provides regular orientation of PES molecules, ensuring transparency, mechanical strength and gas barrier property to the nursing bottle. The raised bottom center will contribute to a more uniform and non-directional stretching of the circular corner region of the bottom, and residual stress concentrated in and around the gate flash in the bottom center is diminished to enhance shock resistance of the bottle. The gate flash located in the raised bottom is protected from directly colliding with floor, ground or the like, thus preventing breakage from instantly spreading out from said gate flash.
The floor-engaging circular bottom portion may preferably be made flat and plane to come into a face contact in its entirety with the floor, to render stable such a standing position of the bottle. Alternatively, the bottom circular portion may be pointed in cross section, to be brought into a linear and annular contact with the floor.